Controlling effects caused by exposure of an implantable medical device to a disruptive energy field

ABSTRACT

Techniques are described for controlling effects caused when an implantable medical device (IMD) is subject to a disruptive energy field. The IMD may include an implantable lead that includes one or more electrodes. The IMD may further include a first component having a parasitic inductance. The IMD may further include a second component having a reactance. In some examples, the reactance of the second component may be selected based on the parasitic inductance of the first component such that an amount of energy reflected along the lead in response to energy produced by an electromagnetic energy source is below a selected threshold. In additional examples, the parasitic inductance of the first component and the reactance of the second component are configured such that an amount of energy reflected along the lead in response to a frequency of electromagnetic energy is below a selected threshold.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/256,804, filed Oct. 30, 2009, and of U.S. Provisional Application No.61/256,794, filed Oct. 30, 2009, the contents of which are herebyincorporated by reference in their entirety.

TECHNICAL FIELD

The disclosure relates to implantable medical devices, and moreparticularly, to controlling effects caused by exposure of animplantable medical device to a disruptive energy field.

BACKGROUND

Some types of implantable medical devices (IMDs) provide therapeuticelectrical stimulation to and/or monitor activity of a tissue of apatient via electrodes of one or more implantable leads that includeelectrodes. Examples of such devices include implantable cardiacpacemakers, cardioverters, defibrillators, neurostimulators, musclestimulators, or the like. In the case of therapeutic electricalstimulation, the electrical stimulation may be delivered to the tissuevia the electrodes of implantable leads in the form of neurostimulationpulses, pacing pulses, cardioversion shocks, defibrillation shocks,cardiac resynchronization or other signals. In some cases, electrodescarried by the implantable leads may be used to sense one or morephysiological signals to monitor the condition of a patient and/or tocontrol delivery of therapeutic electrical stimulation based on thesensed signals.

An IMD may be exposed to a disruptive energy field for any of a numberof reasons. For example, one or more medical procedures may need to beperformed on the patient within whom the IMD is implanted for purposesof diagnostics or therapy. For example, the patient may need to have amagnetic resonance imaging (MRI) scan, computed tomography (CT) scan,electrocautery, diathermy or other medical procedure that produces amagnetic field, electromagnetic field, electric field or otherdisruptive energy field.

The disruptive energy field may induce energy on one or more of theimplantable leads coupled to the IMD, which could alter the operation ofthe IMD. For example, the induced energy may produce lead heating, radiofrequency (RF) rectification, and/or device heating effects, which couldalter the pacing and/or sensing thresholds within the IMD.

SUMMARY

In general, this disclosure is directed to techniques for configuringone or more components within an implantable medical device (IMD) suchthat effects are controlled and/or reduced when the device is subject toa disruptive energy field. In some examples, the disruptive energy fieldmay be a magnetic or radio frequency (RF) field generated by anelectromagnetic energy source. In additional examples, the disruptiveenergy field may be an energy field produced by a medical imagingmodality, such as, e.g., a magnetic resonance imaging (MRI) modality.The energy from the electromagnetic energy source may induce currentflow within electrical components of the lead, which can produce leadheating, RF rectification, device heating and other effects.

Certain components within the IMD, although not primarily considered asinductive and/or capacitive components for conventional implantabledevice design, may nevertheless produce parasitic inductances and/orcapacitances. The parasitic inductances and/or capacitances of suchcomponents may be utilized to deliberately design an electrical networkthat reduces the amount of electromagnetic energy reflected along a leadby the IMD for a given frequency or range of frequencies, and therebyreduce the effects caused by the disruptive energy field such as, e.g.,lead heating, device rectification, or device heating.

When an IMD is placed within the presence of a disruptive energy field,the electromagnetic waves and/or energy associated with the field maypropagate along the length of a lead towards the housing of the IMD. Theelectrical components within the device housing may reflect asubstantial portion of the electromagnetic wave along the lead towardthe electrodes. Because the techniques described in this disclosure mayhelp to reduce the amount of reflected energy in the implanted lead, thetechniques of this disclosure may be used to control the effects causedby placing the device in the presence of disruptive energy field.

In one aspect, the disclosure is directed to a method that includesdetermining a parasitic inductance for a first component within an IMDthat includes a lead that includes one or more electrodes. The methodfurther includes selecting a reactance for a second component within theIMD based on the parasitic inductance such that an amount of energyreflected along the lead in response to energy emitted by anelectromagnetic energy source is below a selected threshold.

In another aspect, the disclosure is directed to an IMD that includes animplantable lead that includes one or more electrodes. The devicefurther includes a first component having a parasitic inductance. Thedevice further includes a second component having a reactance selectedbased on the parasitic inductance such that an amount of energyreflected along the lead in response to energy emitted by anelectromagnetic energy source is below a selected threshold.

In another aspect, the disclosure is directed to a method that includesselecting a frequency of energy emitted by an electromagnetic energysource. The method further includes configuring a parasitic inductanceof a first component and a reactance of a second component within theIMD such that an amount of energy reflected along the lead in responseto the selected frequency of the energy is below the selected threshold.

In another aspect, the disclosure is directed to an IMD that includes animplantable lead that includes one or more electrodes. The devicefurther includes a first component having a parasitic inductance. Thedevice further includes a second component having a reactance, whereinthe parasitic inductance of the first component and the reactance of thesecond component are configured such that an amount of energy reflectedalong the lead in response to a frequency of energy produced by anelectromagnetic energy source is below the a selected threshold.

The details of one or more aspects of the disclosure are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the techniques described in this disclosurewill be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram illustrating an example therapy systemcomprising an implantable medical device (IMD) for deliveringstimulation therapy to a heart of a patient via implantable leadsaccording to this disclosure.

FIG. 2 is a conceptual diagram further illustrating the IMD and leads ofthe system of FIG. 1 in greater detail.

FIG. 3 is a block diagram illustrating an example configuration of anIMD according to this disclosure.

FIG. 4 is a block diagram illustrating active circuitry of FIG. 3 ingreater detail.

FIG. 5 is a circuit diagram illustrating an example electrical networkthat models electrical behavior of various components within an IMDaccording to this disclosure.

FIGS. 6A and 6B are charts illustrating example reflection coefficientplots for an electrical network model according to this disclosure.

FIG. 7 is a flow diagram illustrating an example technique forcontrolling lead heating effects according to this disclosure.

FIG. 8 is a flow diagram illustrating another example technique forcontrolling lead heating effects according to this disclosure.

FIG. 9 is a flow diagram illustrating another example technique forcontrolling lead heating effects according to this disclosure.

FIG. 10 is a flow diagram illustrating an example technique forcontrolling lead heating effects and power rectification according tothis disclosure.

FIG. 11 is a flow diagram illustrating an example technique forcontrolling lead heating effects and device heating effects according tothis disclosure.

FIG. 12 is a flow diagram illustrating an example technique forcontrolling lead heating effects, power rectification, and deviceheating effects according to this disclosure.

DETAILED DESCRIPTION

In general, this disclosure is directed to techniques for configuringone or more components within an implantable medical device (IMD) suchthat effects on the IMD are controlled and/or reduced when the device issubject to a disruptive energy field. In some examples, the disruptiveenergy field may be a magnetic or radio frequency (RF) field generatedby an electromagnetic energy source. In additional examples, thedisruptive energy field may be an energy field produced by a medicalimaging modality, such as a magnetic resonance imaging (MRI) modalityfor example. The energy from the electromagnetic energy source mayinduce current flow within electrical components of the lead, which canproduce lead heating, RF rectification, device heating and othereffects. As used herein, the terms “lead heating” or “lead heatingeffects” may refer to the heating of tissue proximate to one or moreelectrodes coupled to an implantable medical device.

Certain components within the IMD, although not primarily considered asinductive and/or capacitive components for conventional implantabledevice design, may nevertheless produce parasitic inductances and/orcapacitances. The parasitic inductances and/or capacitances of suchcomponents may be utilized to deliberately design an electrical networkthat reduces the amount of electromagnetic energy reflected along a leadby the IMD for a given frequency or range of frequencies, and therebyreduce the effects caused by the disruptive energy field such as, e.g.,lead heating, device rectification, or device heating.

When an IMD is placed within the presence of a disruptive energy field,the electromagnetic waves and/or energy associated with the field maypropagate along the length of a lead towards the housing of the IMD. Theelectrical components within the device housing may reflect asubstantial portion of the electromagnetic wave along the lead towardthe electrodes. In some cases, the reflected electromagnetic wave mayproduce lead heating effects, e.g., tissue heating proximate to theelectrodes. Because the techniques described in this disclosure may helpto reduce the amount of reflected energy in the implanted lead, thetechniques of this disclosure may be used to control the lead heatingeffects caused by placing the device in the presence of disruptiveenergy field.

The electromagnetic energy source may produce electromagnetic energy atany frequency. In examples where the electromagnetic energy source is anMRI modality, the electromagnetic energy source may, in some examples,produce electromagnetic waves or energy (e.g., a disruptive energyfield) having frequencies of 42 MHz, 64 MHz, and/or 128 MHz. Theproportion of 42 MHz radio frequency (RF) to a 1 Tesla (T) staticmagnetic field is controlled by the Larmar frequency. Therefore 42 MHz,64 MHz, and 128 MHz correspond to 1 T, 1.5 T, and 3 T MRIs respectively.However, the techniques in this disclosure may be applied to otherfrequencies of MRIs as well.

In some examples, an IMD in accordance with this disclosure may havecomponents selected such that an amount of reflected energy along adevice lead is below a selected threshold. For example, the IMD mayinclude an implantable lead that carries one or more electrodes, a firstcomponent having a parasitic inductance, and a second component having areactance. In some examples, the reactance of the second component maybe selected based on the parasitic inductance of the first componentsuch that an amount of energy reflected along the lead in response toenergy produced by an electromagnetic energy source is below a selectedthreshold. In additional examples, the parasitic inductance of the firstcomponent and the reactance of the second component are configured suchthat an amount of energy reflected along the lead in response to afrequency of energy emitted by the electromagnetic energy source isbelow the selected threshold.

In some examples, the amount of energy reflected along the lead (i.e.,reflected energy) may refer to a raw amount of reflected energy. As usedherein, a raw amount of reflected energy may refer to the magnitude ofelectromagnetic energy that is reflected at the proximal end of the lead(e.g., at the lead-device interface) in response to electromagneticenergy being induced in the lead by a disruptive electromagnetic field.In some examples, the raw amount of reflected energy may be anelectromagnetic wave that travels towards the distal end of the lead(i.e., away from the lead-device interface).

In additional examples, the amount of energy reflected along the lead(i.e., reflected energy) may refer to a composite amount of reflectedenergy. As used herein, a composite amount of reflected energy may referto the combination (e.g., superposition) of the raw amount of reflectedenergy along the lead and additional amounts of energy traveling alongthe respective lead. The additional amounts of energy may include energythat is reflected by the implantable medical device and/or energy thatis not reflected by the implantable medical device. In some examples,the additional amounts of energy may include energy that is induced inthe respective electrical lead by a disruptive electromagnetic energyfield.

For example, when an implantable medical device is subject to adisruptive electromagnetic energy field, the disruptive electromagneticenergy field may induce a first type of electromagnetic wave in a leadcoupled to the implantable medical device. The first type ofelectromagnetic wave may travel along the lead towards the proximal endof the lead (i.e., towards the lead-device interface of the implantablemedical device). The disruptive electromagnetic energy field may alsoinduce a second type of electromagnetic wave in the lead. The secondtype of electromagnetic wave may travel along the lead towards thedistal end of the lead (i.e., away from the lead-device interface of theimplantable medical device).

The implantable medical device may reflect the first type ofelectromagnetic wave to produce a third type of electromagnetic wave.For example, an electrical network formed by components of theimplantable medical device, as described in further detail in thisdisclosure, may reflect the first type of electromagnetic wave andproduce the third type of electromagnetic wave. The third type ofelectromagnetic wave may travel along the lead towards the distal end ofthe lead (i.e., away from the lead-device interface of the implantablemedical device). The second type of electromagnetic wave and the thirdtype of electromagnetic may combine (e.g., superpose) to form acomposite amount of reflected energy. Thus, in such examples, thecomposite amount of reflected energy may refer to the combination (e.g.,superposition) of the second type of electromagnetic wave and the thirdtype of electromagnetic wave. The techniques in this disclosure may, insome examples, be used to cause the composite amount of energy reflectedalong the lead in response to energy produced by an electromagneticenergy source to be below a selected threshold.

In additional examples, an IMD in accordance with this disclosure mayhave components selected such that one or more of the followingconstraints are satisfied: (1) an amount of reflected energy along adevice lead is below a first threshold; (2) an amount of energytransferred to active circuitry within the medical device is below asecond threshold; and/or (3) an amount of energy dissipated by anelectrical network within the IMD is below a third threshold. Again, asused in this disclosure, the term “reflected energy” may refer to a rawamount of reflected energy or a composite amount of reflected energy.

In some examples, an IMD in accordance with this disclosure may havecomponents selected such that an electrical network that includes thecomponents has a resonant frequency proximate to the frequency of energyproduced by the electromagnetic energy source. In additional examples,an IMD in accordance with this disclosure may have components configuredsuch that a magnitude of the resonance is within a particular range. Infurther examples, an IMD in accordance with this disclosure may havecomponents configured such that the quality factor (Q-factor) orbandwidth of the electrical network is greater than a selectedthreshold.

As used herein, a parasitic inductance refers to internal inductance ofa component whose primary function is something other than behaving asan inductor. In other words, the parasitic inductance is incidental tothe primary function of the component. Similarly, a parasitic resistancerefers to internal resistance of a component whose primary function issomething other than behaving as a resistor. Likewise, a parasiticcapacitance refers to the internal capacitance of a component whoseprimary function is something other than behaving as a capacitor.

In contrast, an actual inductance may refer to the inductance of acomponent whose primary function is to behave as an inductor. Similarly,an actual resistance may refer to the resistance of a component whoseprimary function is to behave as a resistor. Likewise, an actualcapacitance may refer to the capacitance of a component whose primaryfunction is to behave as a capacitor. Any determination of whether aninductance, resistance, or capacitance is parasitic or actual may bebased upon the context and function of the components within a givencircuit.

For purposes of illustration, the techniques of this disclosure will bedescribed with respect to a disruptive energy field generated by animaging modality and, more specifically, a magnetic resonance imaging(MRI) modality. The techniques of this disclosure may, however, be usedin the context of other disruptive energy fields generated by imagingmodalities other than MRI modalities or non-imaging medical ornon-medical devices that generate an energy field.

Although the techniques in this disclosure are described with respect to“energy” or “an amount of energy,” it should be recognized that othermeasures of electromagnetic fields and/or radiation may also be used.For example, “power” or “an amount of power” may be used in place of“energy” and/or “an amount of energy.” In addition, where thisdisclosure refers to “electromagnetic waves,” the disclosure may also bereferring to “electromagnetic energy” and/or “electromagneticradiation.”

FIG. 1 is a conceptual diagram illustrating an example therapy system 10that may be used to provide therapy to heart 12 of patient 14. Patient14 is ordinarily, but not necessarily, a human patient. Therapy system10 includes IMD 16, which is coupled to leads 18, 20, and 22, andprogrammer 24. Although leads 18, 20, 22 are described in FIG. 1 asbeing separate from IMD 16, in some examples, IMD 16 may include leads18, 20, 22.

IMD 16 may be, for example, a device that provides cardiac rhythmmanagement therapy to heart 12, and may include, for example, animplantable pacemaker, cardioverter, and/or defibrillator that providestherapy to heart 12 of patient 14 via electrodes coupled to one or moreof leads 18, 20, and 22. In some examples, IMD 16 may deliver pacingpulses, but not cardioversion or defibrillation shocks, while in otherexamples, IMD 16 may deliver cardioversion and/or defibrillation shocksin addition to pacing pulses. In additional examples, IMD 16 may providecardiac resynchronization therapy in addition to or in lieu of pacingpulses, cardioversion shocks, and/or defibrillation shocks.

Although the techniques in this disclosure are described with respect toan implantable cardiac device for exemplary purposes, such techniquesmay also be applied to other types of IMDs. For example, the techniquesin this disclosure may be applied to neurostimulators, including deepbrain stimulators, spinal cord stimulators, peripheral nervestimulators, pelvic floor stimulators, gastro-intestinal stimulators, orthe like.

Leads 18, 20, 22 extend into the heart 12 of patient 14 to senseelectrical activity of heart 12 and/or deliver electrical stimulation toheart 12. In the example shown in FIG. 1, right ventricular (RV) lead 18extends through one or more veins (not shown), the superior vena cava(not shown), and right atrium 26, and into right ventricle 28. Leftventricular (LV) coronary sinus lead 20 extends through one or moreveins, the vena cava, right atrium 26, and into the coronary sinus 30 toa region adjacent to the free wall of left ventricle 32 of heart 12.Right atrial (RA) lead 22 extends through one or more veins and the venacava, and into right atrium 26 of heart 12. In other examples, therapysystem 10 may include an additional lead or lead segment (not shown inFIG. 1) that deploys one or more electrodes within the vena cava orother vein. These electrodes may allow alternative electrical sensingconfigurations that may provide improved sensing accuracy in somepatients.

IMD 16 senses electrical signals attendant to the depolarization andrepolarization of heart 12 via electrodes (not shown in FIG. 1) coupledto at least one of the leads 18, 20, 22. In some examples, IMD 16provides pacing pulses to heart 12 based on the electrical signalssensed within heart 12. These electrical signals sensed within heart 12may also be referred to as cardiac signals or electrical cardiacsignals. The configurations of electrodes used by IMD 16 for sensing andpacing may be unipolar or bipolar. IMD 16 may also providedefibrillation therapy and/or cardioversion therapy via electrodeslocated on at least one of the leads 18, 20, 22. IMD 16 may detectarrhythmia of heart 12, such as fibrillation of ventricles 28 and 32,and deliver cardioversion or defibrillation therapy to heart 12 in theform of electrical pulses. In some examples, IMD 16 may be programmed todeliver a progression of therapies, e.g., pulses with increasing energylevels, until a tachyarrhythmia of heart 12 is stopped. IMD 16 detectstachycardia or fibrillation employing one or more tachycardia orfibrillation detection techniques known in the art.

In some examples, programmer 24 may be a handheld computing device,computer workstation, or networked computing device. Programmer 24includes a user interface that receives input from a user. The userinterface may include, for example, a keypad and a display, which mayfor example, be a cathode ray tube (CRT) display, a liquid crystaldisplay (LCD) or light emitting diode (LED) display. The keypad may takethe form of an alphanumeric keypad or a reduced set of keys associatedwith particular functions. Programmer 24 can additionally oralternatively include a peripheral pointing device, such as a mouse, viawhich a user may interact with the user interface. In some examples, adisplay of programmer 24 may include a touch screen display, and a usermay interact with programmer 24 via the display. It should be noted thatthe user may also interact with programmer 24 or IMD 16 remotely via anetworked computing device.

A user, such as a physician, technician, surgeon, electrophysiologist,or other clinician, may interact with programmer 24 to communicate withIMD 16. For example, the user may interact with programmer 24 toretrieve physiological or diagnostic information from IMD 16. A user mayalso interact with programmer 24 to program IMD 16, e.g., select valuesfor operational parameters of IMD 16.

For example, the user may use programmer 24 to retrieve information fromIMD 16 regarding the rhythm of heart 12, trends therein over time, ortachyarrhythmia episodes. As another example, the user may useprogrammer 24 to retrieve information from IMD 16 regarding other sensedphysiological parameters of patient 14, such as electricaldepolarization/repolarization signals from the heart (referred to as“electrogram” or EGM), intracardiac or intravascular pressure, activity,posture, respiration, heart rate, heart sounds, or thoracic impedance.As another example, the user may use programmer 24 to retrieveinformation from IMD 16 regarding the performance or integrity of IMD 16or other components of system 10, such as leads 18, 20 and 22, or apower source of IMD 16.

The user may use programmer 24 to program a therapy progression, selectelectrodes used to deliver defibrillation shocks, select waveforms forthe defibrillation shocks, or select or configure a fibrillationdetection algorithm for IMD 16. The user may also use programmer 24 toprogram similar aspects of other therapies provided by IMD 16, such ascardioversion or pacing therapies. In some examples, the user mayactivate certain features of IMD 16 by entering a single command viaprogrammer 24, such as depression of a single key or combination of keysof a keypad or a single point-and-select action with a pointing device.

IMD 16 and programmer 24 may communicate via wireless communicationusing any techniques known in the art. Examples of communicationtechniques may include, for example, low frequency or radiofrequency(RF) telemetry, but other techniques are also contemplated. In someexamples, programmer 24 may include a programming head that may beplaced proximate to the patient's body near the IMD 16 implant site inorder to improve the quality or security of communication between IMD 16and programmer 24.

In accordance with this disclosure, IMD 16 may have one or morecomponents configured such that, when patient 14 and/or IMD 16 are inthe presence of an electromagnetic energy source, the amount of energyreflected by IMD 16 is less than a selected threshold. In some examples,the reactance of a reactive component within IMD 16 may be selectedbased on a parasitic inductance of another component within IMD 16 inorder to control the amount of energy reflected by IMD 16. For example,a capacitance for a channel capacitor within IMD 16 may be selectedbased on a parasitic inductance of ribbon bonding and/or conductivetraces within IMD 16.

Each lead 18, 20, 22 may include one or more electrical conductors. Eachconductor may extend between an electrical contact at a proximal end ofa lead and an electrode at a distal end of the lead. The conductors mayconduct stimulation current to the electrodes and/or conduct sensingcurrent from the electrodes. The electrical contacts may be electricallycoupled to respective electrical terminals in a header associated withthe housing of the IMD 16. The ribbon bonding and/or conductive tracesmay electrically couple the electrical terminals to correspondingelectrical terminals inside the housing of the IMD 16, e.g., on acircuit board that includes various electronic circuit components forgeneration, control and/or processing of electrical stimulation and/orsensing signals. The conductors may carry current that is induced byenergy associated with an electromagnetic energy source, such as an MRIor other imaging modality.

In additional examples, a parasitic inductance value for a firstcomponent and a reactance for a second component may be selected andconfigured in order to control the amount of energy reflected by IMD 16along the length of leads 18, 20, 22 toward the electrodes at the distalends of the leads. For example, the length of ribbon bonding and/orconductive traces within IMD 16 may be adjusted to control the parasiticinductance of the first component. In addition, a capacitance value maybe selected for a channel capacitor within IMD 16 to control thereactance of the second component. The first component and the secondcomponent may operate together to produce a resonance proximate to thefrequency of the electromagnetic energy source.

In some cases, the length of the ribbon bonding and/or conductive tracescan be selected based on a fixed capacitance value of the channelcapacitor to produce a resonance at a desired frequency. In someexamples, the fixed capacitance value may be based on one or morestandard capacitance values for channel capacitors used in a particularmanufacturing process for an IMD or circuit board. Alternatively, thecapacitance value of the channel capacitor may be selected based on afixed inductance value of the ribbon bonding and/or conductive traces toproduce a resonance at a desired frequency. In some examples, the fixedinductance value may be based on a parasitic inductance value associatedwith a particular configuration (e.g., length) of ribbon bonding and/orconductive traces used in a particular manufacturing process for an IMDor circuit board. As a further alternative, the capacitance of thechannel capacitor and the length of the ribbon bonding and/or conductivetraces may both be adjusted or selected to produce a resonance at adesired frequency.

In some examples, the first and second components may be configured suchthat a magnitude of the resonance is within a particular range. Infurther examples, the first and second components may be configured suchthat the quality factor (Q-factor) or bandwidth of an electrical networkcontaining the first and second components is greater than a selectedthreshold.

In further examples, IMD 16 may have components selected such that oneor more of the following constraints are satisfied: (1) an amount ofreflected energy along a device lead is below a first threshold; (2) anamount of energy transferred to active circuitry within the medicaldevice is below a second threshold; and/or (3) an amount of energydissipated by an electrical network within the IMD is below a thirdthreshold.

FIG. 2 is a conceptual diagram illustrating a three-lead IMD 16 andleads 18, 20 and 22 of therapy system 10 in greater detail. Leads 18,20, 22 may be electrically coupled to a signal generator and a sensingmodule of IMD 16 via connector block 34. In some examples, proximal endsof leads 18, 20, 22 may include electrical contacts that electricallycouple to respective electrical contacts within connector block 34 ofIMD 16. In addition, in some examples, leads 18, 20, 22 may bemechanically coupled to connector block 34 with the aid of set screws,connection pins, snap connectors, or another suitable mechanicalcoupling mechanism.

Each of the leads 18, 20, 22 includes an elongated insulative lead body,which may carry a number of concentric coiled conductors separated fromone another by tubular insulative sheaths. Other lead configurations arealso contemplated, such as configurations that do not include coiledconductors. In the illustrated example, bipolar electrodes 40 and 42 arelocated proximate to a distal end of lead 18 in RV 28. In addition,bipolar electrodes 44 and 46 are located proximate to a distal end oflead 20 in LV 32 and bipolar electrodes 48 and 50 are located proximateto a distal end of lead 22 in RA 26. Although no electrodes are locatedin LA 36 in the illustrated example, other examples may includeelectrodes in LA 36.

Electrodes 40, 44, and 48 may take the form of ring electrodes, andelectrodes 42, 46, and 50 may take the form of extendable helix tipelectrodes mounted retractably within insulative electrode heads 52, 54,and 56, respectively. In other examples, one or more of electrodes 42,46, and 50 may take the form of small circular electrodes at the tip ofa tined lead or other fixation element. Leads 18, 20, 22 also includeelongated electrodes 62, 64, 66, respectively, which may take the formof a coil. Each of the electrodes 40, 42, 44, 46, 48, 50, 62, 64, and 66may be electrically coupled to a respective one of the conductors withinthe lead body of its associated lead 18, 20, 22, and thereby coupled torespective ones of the electrical contacts on the proximal end of leads18, 20, 22.

In some examples, as illustrated in FIG. 2, IMD 16 includes one or morehousing electrodes, such as housing electrode 58, which may be formedintegrally with an outer surface of hermetically-sealed housing 60 ofIMD 16 or otherwise coupled to housing 60. In some examples, housingelectrode 58 is defined by an uninsulated portion of an outward facingportion of housing 60 of IMD 16. Other divisions between insulated anduninsulated portions of housing 60 may be employed to define two or morehousing electrodes. In some examples, housing electrode 58 comprisessubstantially all of housing 60. As described in further detail withreference to FIGS. 3 and 4, housing 60 may enclose a signal generatorthat generates therapeutic stimulation, such as cardiac pacing pulsesand defibrillation shocks, as well as a sensing module for monitoringthe rhythm of heart 12.

IMD 16 may sense electrical signals attendant to the depolarization andrepolarization of heart 12 via electrodes 40, 42, 44, 46, 48, 50, 58,62, 64, and 66. The electrical signals are conducted to IMD 16 from theelectrodes via the respective leads 18, 20, 22 or, in the case ofhousing electrode 58, a conductor couple to housing electrode 58. IMD 16may sense such electrical signals via any bipolar combination ofelectrodes 40, 42, 44, 46, 48, 50, 58, 62, 64, and 66. Furthermore, anyof the electrodes 40, 42, 44, 46, 48, 50, 58, 62, 64, and 66 may be usedfor unipolar sensing in combination with housing electrode 58.

Any multipolar combination of two or more of electrodes 40, 42, 44, 46,48, 50, 58, 62, 64, and 66 may be considered a sensing electrodeconfiguration. Usually, but not necessarily, a sensing electrodeconfiguration is a bipolar electrode combination on the same lead, suchas electrodes 40 and 42 of lead 18. On one lead having three electrodes,there may be at least three different sensing electrode configurationsavailable to IMD 16. These sensing electrode configurations are, for theexample of lead 18, tip electrode 42 and ring electrode 40, tipelectrode 42 and elongated electrode 62, and ring electrode 40 andelongated electrode 62. However, some examples may utilize sensingelectrode configurations having electrodes of two different leads.Further, a sensing electrode configuration may utilize housing electrode58, which may provide a unipolar sensing electrode configuration. Insome examples, a sensing electrode configuration may comprise multiplehousing electrodes 58. In any sensing electrode configuration, thepolarity of each electrode in the may be configured as appropriate forthe application of the sensing electrode configuration.

In some examples, IMD 16 delivers pacing pulses via bipolar combinationsof electrodes 40, 42, 44, 46, 48 and 50 to produce depolarization ofcardiac tissue of heart 12. In some examples, IMD 16 delivers pacingpulses via any of electrodes 40, 42, 44, 46, 48 and 50 in combinationwith housing electrode 58 in a unipolar configuration. Furthermore, IMD16 may deliver cardioversion or defibrillation shocks to heart 12 viaany combination of elongated electrodes 62, 64, 66, and housingelectrode 58. Electrodes 58, 62, 64, 66 may also be used to delivercardioversion shocks to heart 12. Electrodes 62, 64, 66 may befabricated from any suitable electrically conductive material, such as,but not limited to, platinum, platinum alloy, Titanium nitride or othermaterials known to be usable in implantable defibrillation electrodes.

As described above, exposure of IMD 16 to a disruptive energy field,e.g., one or more fields produced by an MRI imaging modality, may resultin lead heating, RF rectification, device heating, and/or other effects.For example, RF fields produced by the MRI imaging modality may induceenergy on one or more conductors of respective ones of implantable leads18, 20, or 22 or on the housing electrode, which may in turn increaselead heating, device rectification, and/or device heating effects.According to this disclosure, techniques for configuring one or moreelectrical components within IMD 16 are provided that may reduce and/orcontrol one or more of such effects.

The configuration of therapy system 10 illustrated in FIGS. 1 and 2 ismerely one example. In other examples, a therapy system may includeepicardial leads and/or patch electrodes instead of or in addition tothe implanted leads 18, 20, 22 illustrated in FIG. 1. Further, housing60 of IMD 16 need not be implanted within patient 14. In examples inwhich housing 60 is not implanted in patient 14, IMD 16 may deliverdefibrillation pulses and other therapies to heart 12 via percutaneousleads that extend through the skin of patient 14 to a variety ofpositions within or outside of heart 12.

In other examples of therapy systems that provide electrical stimulationtherapy to heart 12, a therapy system may include any suitable number ofleads coupled to IMD 16, and each of the leads may extend to anylocation within or proximate to heart 12. For example, a therapy systemmay include a single chamber or dual chamber device rather than athree-chamber device as shown in FIG. 1. In a single chamberconfiguration, IMD 16 is electrically connected to a single lead 20 thatincludes stimulation and sense electrodes within LV 32. In one exampleof a dual chamber configuration, IMD 16 is electrically connected to asingle lead that includes stimulation and sense electrodes within LV 32as well as sense and/or stimulation electrodes within RA 26. In anotherexample of a dual chamber configuration, IMD 16 is connected to twoleads that extend into a respective one of the RA 28 and LV 32. Otherlead configurations are contemplated, and the techniques in thisdisclosure are not limited to any particular number of leads orconfiguration of leads.

The techniques of this disclosure may be used to operate an IMD thatprovides other types of electrical stimulation therapy other thancardiac rhythm management therapy or in devices that provide no therapyat all, but only monitor a condition of a patient. For example, the IMDmay be a device that provides electrical stimulation to a tissue site ofpatient 12 proximate a muscle, organ or nerve, such as a tissueproximate a vagus nerve, spinal cord, brain, stomach, pelvic floor orthe like. Moreover, the techniques may be used to operate an IMD thatprovides other types of therapy, such as drug delivery or infusiontherapies. As such, description of these techniques in the context ofcardiac rhythm management therapy should not be limiting of thetechniques as broadly described in this disclosure.

FIG. 3 is a block diagram illustrating an example configuration of IMD16. In the example illustrated by FIG. 3, IMD 16 includes connectorblock 34 and housing 60. Connector block 34 is configured to coupleleads 18, 20, 22 to IMD 16. Connector block 34 includes lead port 70,feedthrough assembly 72 and conductors 74.

Lead port 70 may be configured to secure leads 18, 20, 22 to connectorblock 34. Each of leads 18, 20, 22 may have a lead connector on theproximal end of the lead, which can be inserted into lead port 70. Theproximal lead connector may include electrical contacts, each of whichmay be coupled to a respective electrode at the distal end of the leadvia a respective lead conductor. Lead port 70 may include a suitablemeans for locking the lead connector into lead port 70 such that, wheninserted, each of the electrical contacts is electrically coupled to arespective conductor 74. For example, lead port 70 may include one ormore lead receptacles each of which are configured to receive and lockone of leads 18, 20, 22 in place.

Feedthrough assembly 72 is configured to provide electricalcommunication between the outside and inside of hermetically-sealedhousing 60. Feedthrough assembly may, in some examples, be affixed to aside-wall of housing 60. Feedthrough assembly 72 may include any type ofconductor that provides a conductive pathway between the exterior andinterior of housing 60. Each of the feedthrough conductors may have afirst terminal that is disposed outside of housing 60 and a secondterminal that is disposed inside of housing 60. The first terminal ofeach feedthrough conductor may be electrically coupled to respectiveconductor 74, which is in turn electrically coupled to a respective lead18, 20, 22. A second terminal of each feedthrough conductor may beelectrically coupled to a respective pad 82 on circuit board 76. In someexamples, the feedthrough conductors may include one or more feedthroughpins.

Feedthrough assembly 72 may, in some examples, include one or morefeedthrough capacitors. For each feedthrough capacitor, a first terminalmay be electrically coupled to a portion of a respective feedthroughconductor (e.g., the feedthrough pin) and a second terminal may beelectrically coupled to housing 60. In addition, the first terminal ofthe feedthrough capacitor may be electrically coupled to a respectivecontact of an implantable lead, e.g., via a respective conductor 74. Thefeedthrough capacitors may be configured to block or attenuateelectromagnetic interference (EMI) from entering housing 60 andelectronic circuitry of IMD 16. In some examples, the feedthroughcapacitors may attenuate any incoming high frequency electrical energy,e.g., any frequency of electrical energy that is above approximately 3MHz. Although the feedthrough capacitors are described as being includedwithin feedthrough assembly 72, in other examples, the feedthroughcapacitors may be completely or partially outside of feedthroughassembly 72.

EMI (e.g., a disruptive energy field) may refer to any unwantedelectrical noise (e.g., energy). In other words, EMI may include anyelectromagnetic signal or noise detected by leads 40, 42, 44, 46, 48,50, 58, 62, 64, 66 other than the physiological signals which the leadsare designed to detect. For example, EMI may include electromagneticinterference due to cell phones, microwaves, radios, radar, television,monitors, spark plugs, electric motors, or any other device operatednear IMD 16 that produces electromagnetic energy.

Each of the feedthrough capacitors contained within feedthrough assembly72 may have a capacitance value associated with the capacitor. Asdescribed in further detail in this disclosure, the capacitance valuesfor each of the feedthrough capacitors may be selected and/or adjustedbased on parasitic inductances contained with IMD 16 to control and/orreduce lead heating effects, RF rectification, and/or device heatingeffects caused by a disruptive energy field.

Conductors 74 are configured to provide electrical communication betweenlead port 70 and feedthrough assembly 72. In some examples, eachconductor 74 may be associated with a respective electrical contact inlead port 70 and a respective feedthrough conductor and/or feedthroughcapacitor in feedthrough assembly 72. Thus, for each of the conductors74, a first end or terminal may be electrically coupled to a respectiveelectrical contact within lead port 70 and a second end or terminalelectrically coupled to a feedthrough conductor contained in feedthroughassembly 72. In some examples, each of the conductors 74 may beimplemented as a conductive wire or as an interconnect ribbon.

Housing 60 may be configured to shield the electrical componentscontained inside of housing 60 from body fluids of patient 14 and fromEMI. As described in further detail in this disclosure, electricalcomponents that are partially and/or completely contained within housing60 may be configured such that an amount of energy reflected by IMD 16in response to energy emitted by an electromagnetic energy source isless than a selected threshold.

In some examples, the reactance of a reactive component may beconfigured based on the parasitic inductance of another component. Inadditional examples, the reactance of one component and the parasiticinductance of another component may be configured such that anelectrical network containing the components resonates at a particularfrequency. The resonant frequency may be a frequency that is near (i.e.,proximate) to a frequency of electromagnetic energy produced by anelectromagnetic energy source such that lead heating effects associatedwith the electromagnetic energy source are reduced. For example, theresonant frequency may be proximate to a frequency of electromagneticenergy produced by an MRI or other medical imaging modality. In someexamples, the frequency of the electromagnetic energy may be a centerfrequency of the electromagnetic energy produced by the electromagneticenergy source.

As the distance between the resonant frequency and the frequency ofelectromagnetic energy produced by the electromagnetic energy sourcedecreases, the amount of electrical energy reflected along the length ofa lead by the electrical network may also decrease. Thus, by configuringthe electrical network to resonate a frequency proximate to thefrequency of electromagnetic energy, the amount of current within thelead may be reduced thereby reducing lead heating effects.

As illustrated in FIG. 3, housing 60 includes feedthrough assembly 72,circuit board 76, power source 78, and ribbon bonds 80. Circuit board 76may be a substrate that is used to mechanically support and electricallyconnect electrical components contained on circuit board 76. In someexamples, circuit board 76 may be a printed circuit board (PCB) or aprinted wiring board (PWB). Circuit board 76 may be electrically coupledto feedthrough assembly 72 via ribbon bonds 80. In addition, circuitboard 76 may be electrically coupled to power source 78. Circuit board76 may include one or more pads 82, one or more channel capacitors 84,active circuitry 86, and one or more conductive traces 88, 90. In someexamples, one or more of these components may be affixed to circuitboard 76.

Ribbon bonds 80 are configured to provide electrical communicationbetween feedthrough assembly 72 and circuit board 76. Each of ribbonbonds 80 may have a first end or terminal that is electrically coupledto a respective feedthrough conductor within feedthrough assembly 72,and a second end or terminal that is electrically coupled to arespective pad 82 on printed circuit board 76. In some examples, one ormore of the ribbon bonds 80 may include a laser ribbon bond that islaser-welded to a feedthrough pin at a first end and to a contact pad 82at a second end.

Although ribbon bonds 80 are primarily intended to serve as electricalconductors for delivery of electrical current to and from electrodes onthe leads, each of the ribbon bonds 80 may have a parasitic inductanceassociated with the ribbon bond. As described in further detail in thisdisclosure, these parasitic inductances may be utilized in conjunctionwith other reactive components to control effects (e.g. lead heatingeffects, RF rectification effects, device heating effects, etc.) thatoccur within IMD 16 and leads 18, 20, 22 due to a disruptive energyfield, such as a medical imaging modality.

Pads 82 (e.g., contact pads) are configured to receive an electricalsignal from a component not affixed to circuit board 76 and to relay theelectrical signal to other components affixed to circuit board 76. Foreach pad, a first terminal may be electrically coupled to a respectivefeedthrough conductor in feedthrough assembly 72 via a respective ribbonbond 80, and a second terminal may be electrically coupled to arespective channel capacitor 84 via a respective conductive trace 88.Thus, pads 82 operate as an interface between off-board and on-boardelectrical components.

Channel capacitors 84 may be configured to route stray high-frequencysignals and telemetry signals to a ground voltage terminal (not shown)of circuit board 76. For each channel capacitor 84, a first terminal maybe electrically coupled to a respective conductive trace 88 and arespective conductive trace 90, and a second terminal may beelectrically coupled to a ground voltage terminal of circuit board 76.Each channel capacitor 84 may have a capacitance value associated withthe capacitor. The capacitance value may be selected and/or adjustedbased on parasitic inductances contained with IMD 16 to produce, incombination with other reactive components, a resonant frequency thatserves to control and/or reduce effects caused by a disruptive energyfield.

Active circuitry 86 is configured to control therapy and measurementoperations of IMD 16. Active circuitry 86 may also include any othercomponents or sub-circuits typically included within an IMD. Asdescribed in further detail with respect to FIG. 4, active circuitry mayinclude a diode protection array that protects the active circuitrycomponents. Active circuitry 86 may be electrically coupled to channelcapacitors 84 via conductive traces 90.

Conductive traces 88, 90 are configured to interconnect variouselectrical components within circuit board 76. In some examples, circuitboard 76 may comprise a non-conductive substrate, and the conductivetraces 88, 90 may be formed by etching conductive copper sheets that arelaminated on the non-conductive substrate. Conductive traces 88 mayinclude one or more conductive traces, each of which has a first portionor terminal that is electrically coupled to a respective pad 82 and asecond portion or terminal that is electrically coupled to a respectivechannel capacitor 84. Conductive traces 90 may include one or moreconductive traces, each of which has a first portion or terminal that iselectrically coupled to a respective channel capacitors 84 and a secondportion or terminal that is electrically coupled to a respectiveterminal of active circuitry 86

In some examples, the terminal of active circuitry 84 may be a firstterminal of a diode within active circuitry, and the second terminal ofthe diode may be electrically coupled to a ground terminal of circuitboard 76. Each of the conductive traces 88, 90 may have a parasiticinductance associated with the trace. As described in further detail inthis disclosure, these parasitic inductances may be utilized inconjunction with other reactive components to control and/or reduce leadheating effects due to an electromagnetic energy source that producesEMI.

As used herein, a component of IMD 16 may refer to any electricalcomponent within IMD 16 including conductors 74, channel capacitors 84,active circuitry 86, conductive traces 88, 90, feedthrough capacitorscontained within feedthrough assembly 72, or any other electricalcomponent that is partially or completely contained within housing 60,any component that is partially or completely contained within connectorblock 34 of IMD 16, or any component that is electrically coupled to IMD16.

IMD 16 may include a plurality of channels, each of which may beassociated with a respective electrode 40, 42, 44, 46, 48, 50, 58, 62,64, 66. In other words, each channel within IMD 16 may be associatedwith a conductive pathway that provides electrical communication betweena terminal within IMD 16 and a respective electrode 40, 42, 44, 46, 48,50, 58, 62, 64, 66. As described above with respect to FIG. 3, eachconductive pathway may have a respective conductor 74, a respectivefeedthrough conductor within feedthrough assembly 72, a respectivefeedthrough capacitor, a respective ribbon bond 80, a respective pad 82,a respective conductive trace 88, a respective channel capacitor 84, arespective conductive trace 90, and a respective terminal of activecircuitry 86.

Power source 78 is configured to supply power to one or more of thecomponents within IMD 16. Power source 78 may include a rechargeable ornon-rechargeable battery. A non-rechargeable battery may be capable ofholding a charge for several years, while a rechargeable battery may beinductively charged from an external device, e.g., on a daily or weeklybasis. Examples of a rechargeable battery include, but are not limitedto, a lithium ion battery, a lithium polymer battery or asupercapacitor. Each of the components within IMD 16 may be electricallycoupled to power source 78.

FIG. 4 is a block diagram illustrating the active circuitry 86 of FIG. 4in greater detail. Active circuitry 86 may include diode protectionarray 88, processor 92, memory 94, signal generator 96, electricalsensing module 98, telemetry module 100, and conductive traces 102.

Diode protection array 88 may be configured to prevent excess voltagefrom entering components within active circuitry 96. Diode protectionarray 88 may include one or more diodes. Each of the diodes may have afirst terminal that is electrically coupled to a respective terminal ofsignal generator 96 and a corresponding respective terminal of sensingmodule 98 via a respective conductive trace 102. The first terminal ofeach diode may also be electrically coupled to a respective channelcapacitor 84. Each of the diodes within diode protection array 88 mayhave a second terminal electrically coupled to a ground voltage ofcircuit board 76. The one or more diodes within diode protection array88 may be any combination of forward-biased diodes and/orreversed-biased diodes. In some examples, each channel may include asingle diode, such as a zener diode for example. In other examples, eachchannel may include a plurality of diodes.

In cases where a high-frequency signal reaches diode protection array 88via conductive traces 90, rather than preventing excess voltage fromentering components, the diodes may produce a rectified version of thehigh-frequency signal that is transferred to active circuitry 86. Thisphenomenon may be referred to as “radio frequency (RF) rectification.”According to the techniques in this disclosure, the amount of RFrectification that occurs when IMD 16 is subject to a disruptive energyfield may be controlled. For example, the components within IMD 16 maybe configured such that the amount of energy reflected along the lead inresponse to energy emitted by an electromagnetic energy source is lessthan a first threshold, and the amount of energy transferred to theactive circuitry is below a second threshold. For example, an effectiveresistance (e.g., real impedance) of an electrical network that includeschannel capacitors 84, ribbon bond 80 and/or conductive traces 88, 90may be increased to increase the amount of energy dissipated by theelectrical network. By increasing the effective resistance of theelectrical network, lead heating effects due to an EMI produced by anelectromagnetic energy source may be reduced without causing aprohibitive increase in the amount of RF rectification.

As described above with respect to FIG. 3, each channel within IMD 16may be associated with a conductive pathway that includes a respectiveconductor 74, a respective feedthrough conductor within feedthroughassembly 72, a respective feedthrough capacitor, a respective ribbonbond 80, a respective pad 82, a respective conductive trace 88, arespective channel capacitor 84, a respective conductive trace 90, and arespective terminal of active circuitry 86. In addition, the conductivepathway may also include a respective terminal within diode protectionarray 88, a respective set of one or more diodes within diode protectionarray 88, a respective terminal within signal generator 96, and arespective terminal within sensing module 98. Thus, each channel withinIMD 16 may be associated with or comprise a conductive pathway from anindividual electrode 40, 42, 44, 46, 48, 50, 58, 62, 64, 66 to arespective terminal of diode protection array 88, signal generator 96,and/or sensing module 98.

Processor 92 may include any one or more of a microprocessor, acontroller, a digital signal processor (DSP), an application specificintegrated circuit (ASIC), a field-programmable gate array (FPGA), orequivalent discrete or integrated logic circuitry. In some examples,processor 92 may include multiple components, such as any combination ofone or more microprocessors, one or more controllers, one or more DSPs,one or more ASICs, or one or more FPGAs, as well as other discrete orintegrated logic circuitry. The functions attributed to processor 92herein may be embodied as software, firmware, hardware or anycombination thereof.

Processor 92 controls signal generator 96 to deliver stimulation therapyto heart 12. Processor 92 may control signal generator 96 to deliverstimulation according to a selected one or more therapy programs, whichmay be stored in memory 94. For example, processor 92 may control signalgenerator 96 to deliver electrical pulses with the amplitudes, pulsewidths, frequencies, or electrode polarities specified by the selectedone or more therapy programs.

Memory 94 may include computer-readable instructions that, when executedby processor 92, cause IMD 16 and processor 92 to perform variousfunctions attributed to IMD 16 and processor 92 herein. Memory 94 mayinclude any volatile, non-volatile, magnetic, optical, or electricalmedia, such as a random access memory (RAM), read-only memory (ROM),non-volatile RAM (NVRAM), static RAM (SRAM), electrically-erasableprogrammable ROM (EEPROM), flash memory, or any other digital media.

Signal generator 96 is configured to generate and deliver electricalstimulation therapy to heart 12. Signal generator 96 may deliver pacingpulses via ring electrodes 40, 44, 48 coupled to leads 18, 20, and 22,respectively, and/or helical electrodes 42, 46, and 50 of leads 18, 20,and 22, respectively. For example, signal generator 96 may deliver apacing stimulus to LV 32 (FIG. 2) of heart 12 via at least twoelectrodes 44, 46 (FIG. 2). As another example, signal generator 96 maydeliver defibrillation shocks to heart 12 via at least two electrodes58, 62, 64, 66. In some examples, signal generator 96 delivers pacing,cardioversion, or defibrillation stimulation in the form of electricalpulses. In other examples, signal generator 96 may deliver one or moreof these types of stimulation in the form of other signals, such as sinewaves, square waves, or other substantially continuous time signals.

In some examples, signal generator 96 may include a switch module andprocessor 92 may use the switch module to select, e.g., via adata/address bus, which of the available electrodes are used to deliverpacing pulses, cardioversion shocks, or defibrillation shocks. Theswitch module may include a switch array, switch matrix, multiplexer, orany other type of switching device suitable to selectively couplestimulation energy to selected electrodes. In other examples, however,signal generator 96 may independently deliver stimulation to electrodes40, 42, 44, 46, 48, 50, 58, 62, 64, 66 or selectively sense via one ormore of electrodes 40, 42, 44, 46, 48, 50, 58, 62, 64, 66 without aswitch matrix.

Signal generator 96 may include a plurality of terminals, each of whichis electrically coupled to a first terminal of a respective diode withindiode protection array 88. The second terminal of each of the diodes maybe electrically coupled to a ground voltage terminal of circuit board76. Since the diode protection array is electrically coupled tofeedthrough assembly 72 and lead port 70, each of the terminals ofsignal generator may be electrically coupled to a respective one ofelectrodes 40, 42, 44, 46, 48, 50, 58, 62, 64, and 66, e.g., viaconductors of the respective lead 18, 20, 22, or, in the case of housingelectrode 58, via an electrical conductor disposed within housing 60 ofIMD 16.

Sensing module 98 is configured to monitor signals from at least one ofelectrodes 40, 42, 44, 46, 48, 50, 58, 62, 64, 66 in order to monitorelectrical activity of heart 12. For example, sensing module 98 maysense atrial events (e.g., a P-wave) with electrodes 48, 50, 66 withinRA 26 or sense an LV 32 event (e.g., an R-wave) with electrodes 44, 46,64 within LV 32. Sensing module 98 may also include a switch module toselect which of the available electrodes are used to sense the heartactivity. In some examples, processor 92 may select the electrodes thatfunction as sense electrodes, or the sensing electrode configuration,via the switch module within electrical sensing module 98, e.g., byproviding signals via a data/address bus. In some examples, sensingmodule 98 may include multiple sensing channels, each of which maycomprise an amplifier. In response to the signals from processor 92, theswitch module of within sensing module 98 may couple the outputs fromthe selected electrodes to one or more of the sensing channels.

In some examples, sensing module 98 may include a plurality of channels.One channel of sensing module 98 may include an R-wave amplifier thatreceives signals from electrodes 40 and 42, which are used for pacingand sensing in RV 28 of heart 12. Another channel may include anotherR-wave amplifier that receives signals from electrodes 44 and 46, whichare used for pacing and sensing proximate to LV 32 of heart 12. In someexamples, in one operating mode of sensing module 98, the R-waveamplifiers may take the form of an automatic gain controlled amplifierthat provides an adjustable sensing threshold as a function of themeasured R-wave amplitude of the heart rhythm.

In addition, in some examples, one channel of sensing module 98 mayinclude a P-wave amplifier that receives signals from electrodes 48 and50, which are used for pacing and sensing in right atrium 26 of heart12. In some examples, in one operating mode of sensing module 98, theP-wave amplifier may take the form of an automatic gain controlledamplifier that provides an adjustable sensing threshold as a function ofthe measured P-wave amplitude of the heart rhythm. Examples of R-waveand P-wave amplifiers are described in U.S. Pat. No. 5,117,824 to Keimelet al., which issued on Jun. 2, 1992 and is entitled, “APPARATUS FORMONITORING ELECTRICAL PHYSIOLOGIC SIGNALS,” and is incorporated hereinby reference in its entirety. Other amplifiers may also be used.Furthermore, in some examples, one or more of the sensing channels ofsensing module 98 may be selectively coupled to housing electrode 58, orelongated electrodes 62, 64, or 66, with or instead of one or more ofelectrodes 40, 42, 44, 46, 48 or 50, e.g., for unipolar sensing ofR-waves or P-waves in any of chambers 26, 28, or 32 of heart 12.

In some examples, sensing module 98 may include a channel that comprisesan amplifier with a relatively wider pass band than the R-wave or P-waveamplifiers. Signals from the selected sensing electrodes that areselected for coupling to this wide-band amplifier may be provided to amultiplexer, and thereafter converted to multi-bit digital signals by ananalog-to-digital converter for storage in memory 94 as an EGM. In someexamples, the storage of such EGMs in memory 94 may be under the controlof a direct memory access circuit. Processor 92 may employ digitalsignal analysis techniques to characterize the digitized signals storedin memory 94 to detect and classify the patient's heart rhythm from theelectrical signals. Processor 92 may detect and classify the heartrhythm of patient 14 by employing any of the numerous signal processingmethodologies known in the art.

In some examples, processor 92 may also include programmable counterswhich control the basic time intervals associated with DDD, VVI, DVI,VDD, AAI, DDI, DDDR, VVIR, DVIR, VDDR, AAIR, DDIR and other modes ofsingle and dual chamber pacing. In the aforementioned pacing modes, “D”may indicate dual chamber, “V” may indicate a ventricle, “I” mayindicate inhibited pacing (e.g., no pacing), and “A” may indicate anatrium. The first letter in the pacing mode may indicate the chamberthat is paced, the second letter may indicate the chamber in which anelectrical signal is sensed, and the third letter may indicate thechamber in which the response to sensing is provided.

In some examples, processor 92 may include atrial and ventricular pacingescape intervals, refractory periods during which sensed P-waves andR-waves are ineffective to restart timing of the escape intervals, andthe pulse widths of the pacing pulses. As another example, pacer timingand control module 92 may define a blanking period, and provide signalsfrom sensing module 98 to blank one or more channels, e.g., amplifiers,for a period during and after delivery of electrical stimulation toheart 12. The durations of these intervals may be determined byprocessor 92 in response to stored data in memory 94. The pacer timingand control module of processor 92 may also determine the amplitude ofthe cardiac pacing pulses.

During pacing, escape interval counters within pacer timing and controlmodule 92 of processor 92 may be reset upon sensing of R-waves andP-waves with detection channels of electrical sensing module 98. Signalgenerator 96 may include pacer output circuits that are coupled, e.g.,selectively by a switching module, to any combination of electrodes 40,42, 44, 46, 48, 50, 58, 62, or 66 appropriate for delivery of a bipolaror unipolar pacing pulse to one of the chambers of heart 12. Processor92 may reset the escape interval counters upon the generation of pacingpulses by signal generator 96, and thereby control the basic timing ofcardiac pacing functions, including anti-tachyarrhythmia pacing.

The value of the count present in the escape interval counters whenreset by sensed R-waves and P-waves may be used by processor 92 tomeasure the durations of R-R intervals, P-P intervals, P-R intervals andR-P intervals, which are measurements that may be stored in memory 94.Processor 92 may use the count in the interval counters to detect atachyarrhythmia event, such as ventricular fibrillation event orventricular tachycardia event. Upon detecting a threshold number oftachyarrhythmia events, processor 92 may identify the presence of atachyarrhythmia episode, such as a ventricular fibrillation episode, aventricular tachycardia episode, or a non-sustained tachycardia (NST)episode. Examples of tachyarrhythmia episodes that may qualify fordelivery of responsive therapy include a ventricular fibrillationepisode or a ventricular tachyarrhythmia episode.

In some examples, processor 92 may operate as an interrupt driven devicethat is responsive to interrupts from pacer timing and control module92, where the interrupts may correspond to the occurrences of sensedP-waves and R-waves and the generation of cardiac pacing pulses. Anynecessary mathematical calculations to be performed by processor 92 andany updating of the values or intervals controlled by pacer timing andcontrol module 92 of processor 92 may take place following suchinterrupts. A portion of memory 94 may be configured as a plurality ofrecirculating buffers, capable of holding a series of measuredintervals, which may be analyzed by processor 92 in response to theoccurrence of a pace or sense interrupt to determine whether thepatient's heart 12 is presently exhibiting atrial or ventriculartachyarrhythmia.

In some examples, an arrhythmia detection method may include anysuitable tachyarrhythmia detection algorithms. In one example, processor92 may utilize all or a subset of the rule-based detection methodsdescribed in U.S. Pat. No. 5,545,186 to Olson et al., entitled,“PRIORITIZED RULE BASED METHOD AND APPARATUS FOR DIAGNOSIS AND TREATMENTOF ARRHYTHMIAS,” which issued on Aug. 13, 1996, in U.S. Pat. No.5,755,736 to Gillberg et al., entitled, “PRIORITIZED RULE BASED METHODAND APPARATUS FOR DIAGNOSIS AND TREATMENT OF ARRHYTHMIAS,” which issuedon May 26, 1998, or in U.S. patent application Ser. No. 10/755,185,filed Jan. 8, 2004 by Kevin T. Ousdigian, entitled “REDUCINGINAPPROPRIATE DELIVERY OF THERAPY FOR SUSPECTED NON-LETHAL ARRHYTHMIAS.”U.S. Pat. No. 5,545,186 to Olson et al., U.S. Pat. No. 5,755,736 toGillberg et al., and U.S. patent application Ser. No. 10/755,185 byKevin T. Ousdigian, filed Jan. 8, 2004, are incorporated herein byreference in their entireties. However, other arrhythmia detectionmethodologies may also be employed by processor 92 in other examples.

In additional examples, processor 92 may identify the presence of anatrial or ventricular tachyarrhythmia episode by detecting a series oftachyarrhythmia events (e.g., R-R or P-P intervals having a durationless than or equal to a threshold) of an average rate indicative oftachyarrhythmia or an unbroken series of short R-R or P-P intervals. Thethresholds for determining the R-R or P-P interval that indicates atachyarrhythmia event may be stored within memory 94 of IMD 16. Inaddition, the number of tachyarrhythmia events that are detected toconfirm the presence of a tachyarrhythmia episode may be stored as anumber of intervals to detect (NID) threshold value in memory 94. Insome examples, processor 92 may also identify the presence of thetachyarrhythmia episode by detecting a variable coupling intervalbetween the R-waves of the heart signal. For example, if the intervalbetween successive tachyarrhythmia events varies by a particularpercentage or the differences between the coupling intervals are higherthan a given threshold over a predetermined number of successive cycles,processor 92 may determine that the tachyarrhythmia is present.

In the event that processor 92 detects an atrial or ventriculartachyarrhythmia based on signals from electrical sensing module 98, andan anti-tachyarrhythmia pacing regimen is desired, timing intervals forcontrolling the generation of anti-tachyarrhythmia pacing therapies bysignal generator 96 may be loaded by processor 92 into pacer timing andcontrol module 92 to control the operation of the escape intervalcounters therein and to define refractory periods during which detectionof R-waves and P-waves is ineffective to restart the escape intervalcounters.

If IMD 16 is configured to generate and deliver defibrillation shocks toheart 12, signal generator 96 may include a high voltage charge circuitand a high voltage output circuit. In the event that generation of acardioversion or defibrillation shock is required, processor 92 mayemploy the escape interval counter to control timing of suchcardioversion and defibrillation shocks, as well as associatedrefractory periods. In response to the detection of atrial orventricular fibrillation or tachyarrhythmia requiring a cardioversionpulse, processor 92 may activate a cardioversion/defibrillation controlmodule, which may, like pacer timing and control module 92, be ahardware component of processor 92 and/or a firmware or software moduleexecuted by one or more hardware components of processor 92. Thecardioversion/defibrillation control module may initiate charging of thehigh voltage capacitors of the high voltage charge circuit of signalgenerator 96 under control of a high voltage charging control line.

Processor 92 may monitor the voltage on the high voltage capacitor,e.g., via a voltage charging and potential (VCAP) line. In response tothe voltage on the high voltage capacitor reaching a predetermined valueset by processor 92, processor 92 may generate a logic signal thatterminates charging. Thereafter, timing of the delivery of thedefibrillation or cardioversion pulse by signal generator 96 iscontrolled by the cardioversion/defibrillation control module ofprocessor 92. Following delivery of the fibrillation or tachycardiatherapy, processor 92 may return signal generator 96 to a cardiac pacingfunction and await the next successive interrupt due to pacing or theoccurrence of a sensed atrial or ventricular depolarization.

Signal generator 96 may deliver cardioversion or defibrillation shockswith the aid of an output circuit that determines whether a monophasicor biphasic pulse is delivered, whether housing electrode 58 serves ascathode or anode, and which electrodes are involved in delivery of thecardioversion or defibrillation shocks. Such functionality may beprovided by one or more switches or a switching module of signalgenerator 96.

Telemetry module 100 is configured to receive downlink telemetry fromand send uplink telemetry to programmer 24 with the aid of an antenna,which may be internal and/or external. Telemetry module 100 may becontrolled by processor 92. Telemetry module 100 includes any suitablehardware, firmware, software or any combination thereof forcommunicating with another device, such as programmer 24 (FIG. 1).Processor 92 may provide the data to be uplinked to programmer 24 andthe control signals for the telemetry circuit within telemetry module100, e.g., via an address/data bus. In some examples, telemetry module100 may provide received data to processor 92 via a multiplexer.

In some examples, processor 92 may transmit atrial and ventricular heartsignals (e.g., EGM signals) produced by atrial and ventricular sense ampcircuits within electrical sensing module 98 to programmer 24.Programmer 24 may interrogate IMD 16 to receive the EGMs. Processor 92may store EGMs within memory 94, and retrieve stored EGMs from memory94. Processor 92 may also generate and store marker codes indicative ofdifferent cardiac events that electrical sensing module 98 detects, suchas ventricular and atrial depolarizations, and transmit the marker codesto programmer 24. An example pacemaker with marker-channel capability isdescribed in U.S. Pat. No. 4,374,382 to Markowitz, entitled, “MARKERCHANNEL TELEMETRY SYSTEM FOR A MEDICAL DEVICE,” which issued on Feb. 15,1983 and is incorporated herein by reference in its entirety.

FIG. 5 is a circuit diagram illustrating an example electrical network120 that models electrical behavior of various components within IMD 16.Electrical network 120 includes feedthrough capacitance 122, inductance124, resistance 126, channel capacitance 128, resistance 130, inputterminal nodes 132A, 132B, node 134, and output terminal nodes 136A,136B.

In some examples, electrical network 120 may model electrical behaviorfor a particular channel within IMD 16. As already described above, achannel may be associated with a conductive pathway from a terminalwithin IMD 16 to a respective electrode 40, 42, 44, 46, 48, 50, 58, 62,64, 66. Thus, each channel within IMD 16 may be modeled by an electricalnetwork similar to electrical network 120 illustrated in FIG. 5. In someexamples, each channel may include an identical electrical network. Inother examples, the component values within the electrical networks mayvary from channel-to-channel. For example, the component values for eachchannel may be selected based on the implanted location (e.g., region ofthe heart) of the associated electrode.

Input terminal node 132A may be electrically coupled to one or more ofleads 18, 20, 22 and/or electrodes 40, 42, 44, 46, 48, 50, 58, 62, 64,and 66. In some examples, input terminal node 132 may be electricallycoupled to a feedthrough pin in feedthrough assembly 72. Input terminalnode 132B may be electrically coupled to housing 60 of IMD 16. Outputterminal node 136A may be electrically coupled to active circuitry 86.Output terminal node 136B may be electrically coupled to a groundvoltage terminal of circuit board 76. In some cases, output terminalnode 136B may be capacitively coupled to input terminal node 132B.

Feedthrough capacitance 122 may correspond to the capacitance of one ormore of the feedthrough capacitors located within feedthrough assembly72. Feedthrough capacitance 122 has a capacitance value of C₁.Feedthrough capacitance 122 has a first terminal that is electricallycoupled to input terminal 132A, and a second terminal that iselectrically coupled to input terminal 132B.

Inductance 124 may correspond to the parasitic and/or actual inductanceof one or more electrical components within IMD 16. In some examples,inductance 124 may include a parasitic inductance attributed to anycombination of the following components: (1) ribbon bonds 80; (2)conductive traces 88; and/or (3) conductive traces 90. The actual orparasitic inductance of other components may be included withininductance 124. For example, other components upstream of channelcapacitors 84 may contribute to inductance 124 including the parasiticinductance of feedthrough conductors and/or interconnect ribbons 74.

In any case, an inductance value of L may represent the combinedparasitic or actual inductance of any components included withininductance 124. Inductance 124 has a first terminal that is electricallycoupled to input terminal node 132A, and a second terminal that iselectrically coupled to node 134.

Resistance 126 may correspond to the parasitic and/or actual resistanceof one or more electrical components within IMD 16. Similar toinductance 124, resistance 126 may, in some examples, include aparasitic resistance attributed to any combination of the followingcomponents: (1) ribbon bonds 80; (2) conductive traces 88; and/or (3)conductive traces 90. In additional examples, the parasitic or actualresistance of other components may be included within resistance 126.The parasitic resistance value of R₁ may represent the combined actualor parasitic resistance of any components included within resistance126. Resistance 126 has a first terminal that is electrically coupled tonode 134, and a second terminal that is electrically coupled to outputterminal node 136A.

In additional examples, electrical network 120 may have an additionaldiscrete resistor and/or resistive component that is placed in serieswith resistance 126. In other words, one of the terminals of thediscrete resistor and/or resistance may be electrically coupled to oneof the terminals of resistor 126. In such cases, the resistance value R₁may correspond to an effective resistance that is the combination of theparasitic resistance and the actual resistance of the additionalresistance component.

Channel capacitance 128 may correspond to the capacitance of one or moreof the channel capacitors 84 located on circuit board 76. Channelcapacitance 128 has a capacitance value of C₂. Channel capacitance 128has a first terminal that is electrically coupled to output terminal136A, and a second terminal that is electrically coupled to outputterminal 136B.

In some examples, electrical network 120 may have a parasiticcapacitance between nodes 136A and 136B that is in parallel to channelcapacitance 128. In such cases, the capacitance value C₂ of channelcapacitance 128 may be an effective capacitance based on the actualchannel capacitance 128 and the parallel parasitic capacitance.

Resistance 130 may correspond to the load resistance of one or morecomponents within active circuitry 86. In some examples, resistance 130may include the actual and/or parasitic resistance of one or more diodesin diode protection array 88. In additional examples, the parasitic oractual resistance of other components within active circuitry 86 may beincluded within resistance 130. The resistance value of R₂ may representthe combined parasitic resistance or actual resistance of any componentsincluded within active circuitry 86.

In some examples, electrical network 120 may have an additional discreteresistor and/or resistive component that has a first terminalelectrically coupled to node 136A and a second terminal electricallycoupled to node 136B. In other words, the additional discrete resistorand/or resistance may be in parallel with resistance 130. The additionalresistance may be configured to shape the filtering characteristics ofelectrical network 120.

In some examples, the combined inductance and capacitance of electricalnetwork 120 controls the resonant frequency of electrical network 120.In additional examples, the combined resistance of electrical network120 controls the Q-factor and/or frequency bandwidth of electricalnetwork 120.

In one non-limiting example, the capacitance value of feedthroughcapacitance 122 may be approximately 1.5 nanofarads (nF), the inductancevalue of inductance 124 may be approximately 6 nanohenries (nH), theresistance value of the resistance 126 may be 0.8 ohms (Ω), thecapacitance value of the channel capacitance 128 may be 3.3 nF, and theresistance value of resistance 130 may be 1 kiloohm (kΩ). Theseparameter values are provided for exemplary purposes, and various othercombinations of parameter values are within the scope of thisdisclosure.

In additional examples, a source impedance may be coupled between aninput node 136A of electrical network 120 and a respective electrode 40,42, 44, 46, 48, 50, 58, 62, 64, 66. The source impedance may model theresistance or characteristic impedance of the lead conductor. One ormore of the electrical components within electrical network 120 may beselected based on the characteristic impedance. For example, one or moreelectrical components may be selected such that the difference betweenthe source impedance and the input resistance/impedance of electricalnetwork 120 is below a selected threshold.

As used herein, a parasitic inductance refers to the internal inductanceof a component whose primary function is something other than behavingas an inductor. In other words, the parasitic inductance is incidentalto the primary function of the component. For example, the primaryfunction of the ribbon bonds 80 and the conductive traces 88, 90, may insome cases, be to provide a conductive pathway between two componentswithin IMD 16. In such examples, any inductance produced across theterminals of these components is incidental to the primarypurpose/function of these components as conductors. Thus, any suchinductance may be referred to as a parasitic inductance. Similarly, aparasitic resistance refers to the internal resistance of a componentwhose primary function is something other than behaving as a resistor.Likewise, a parasitic capacitance refers to the internal capacitance ofa component whose primary function is something other than behaving as acapacitor.

In contrast, an actual inductance may refer to the inductance of acomponent whose primary function is to behave as an inductor. Forexample, a lumped inductor may, in some examples, have the primaryfunction of behaving like an inductor. In such examples, the inductancewould be referred to as an actual inductance. Similarly, an actualresistance may refer to the resistance of a component whose primaryfunction is to behave as a resistor. Likewise, an actual capacitance mayrefer to the capacitance of a component whose primary function is tobehave as a capacitor. Any determination of whether an inductance,resistance, or capacitance is parasitic or actual may be based upon thecontext and function of the components within a given circuit.

A reactive component may refer to a component that has an actualreactance (i.e., a component whose primary function is to produce areactance between its terminals). For example, a capacitor may have theprimary function of producing a capacitance between its terminals, andan inductor may the primary function of producing an inductance betweenits terminals. The reactance type for a reactive component may refer towhether a component is an inductor or a capacitor. The reactance type ofa reactance may refer to whether the reactance is inductive orcapacitive. Accordingly, a parasitic reactance, as used herein, mayrefer to a reactance produced by a reactive component where thereactance type of the reactance is different from the reactance type ofthe component itself. For example, a parasitic inductance may refer toan inductance produced by a capacitor. Similarly, a parasiticcapacitance may refer to a capacitance produced by an inductor.

According to this disclosure, several different techniques are providedfor selecting parameter values corresponding to feedthrough capacitor122, inductance 124, resistance 126, channel capacitance 128 and/orresistance 130. These parameter values may be configured to control leadheating effects, RF rectification, device heating and/or other effectscaused by a disruptive energy field proximate to IMD 16.

In some examples, the parasitic inductance 124 may be determined for oneor more components, such as, e.g., ribbon bond 80 and/or conductivetrace 88. Then, a channel capacitance 128 may be selected based on theparasitic inductance 124. The selected channel capacitance 128 and theparasitic inductance 124 may work together to cause electrical network120 to resonate at a resonant frequency.

In additional examples, a reactance may be determined for one or morecomponents, such as, e.g., feedthrough capacitance 122 and/or channelcapacitance 128. Then, a parasitic inductance of one or more componentsmay be configured based on the reactance. In some examples, theparasitic inductance may be configured by selecting a length of ribbonbond 80 and/or conductive traces 88. In additional examples, theparasitic inductance may be configured by selecting a width or diameterof ribbon bond 80 and/or conductive trances 88. In further examples, acombination of length and width/diameter may be used to adjust theparasitic inductance. The channel capacitance 128 and the configuredinductance 124 may work together to cause electrical network 120 toresonate at a resonant frequency. In further examples, the reactance forone or more components and the parasitic inductance for one or morecomponents may be simultaneously configured such that the componentswork together to cause electrical network 120 to resonate at a givenfrequency.

In general, the resonant frequency may refer to a frequency where theeffective input impedance of electrical network 120 matches theeffective impedance of a lead 18, 20, 22 that is electrically coupled toinput terminals 132A, 132B. The components within electrical network 120may be configured such that the resonant frequency is proximate to afrequency of the electromagnetic energy source and/or imaging modality.In some examples, the resonant frequency may be substantially equal tothe frequency of the electromagnetic energy source and/or imagingmodality. In other examples, the resonant frequency may be offset fromthe frequency of the electromagnetic energy source and/or imagingmodality.

In some examples, one or both of the channel capacitance 128 andinductance 124 parameter values may be configured based on the followingequation:

$\begin{matrix}{f_{R} = \frac{1}{2\pi\sqrt{L \cdot C_{2}}}} & (1)\end{matrix}$

where L is the inductance 124, C₂ is the channel capacitance 128, andf_(R) is the resonant frequency. In general, the techniques of thisdisclosure allow for any parameter to be selected if the other twoparameters are known or are defined as constraints. For example, if adesired resonant frequency and an inductance 124 are specified, then thechannel capacitance 128 may be determined based on equation (1). Thetechniques in this disclosure also allow for any two parameters to beselected if the remaining parameter is known or defined as a constraint.For example, if a desired resonant frequency is specified, theninductance 124 and channel capacitance 128 may be determined based onequation (1).

In additional examples, one or more of the feedthrough capacitance 122,channel capacitance 128 and inductance 124 parameter values may beconfigured based on the following equations:

$\begin{matrix}{{f_{R} = \frac{1}{2\pi\sqrt{L \cdot C}}},{C = \frac{C_{1} \cdot C_{2}}{C_{1} + C_{2}}}} & (2)\end{matrix}$

where L is the inductance 124, C₁ is the feedthrough capacitance, C₂ isthe channel capacitance 128, C is the effective capacitance of thefeedthrough capacitance and channel capacitance, and f_(R) is theresonant frequency. Similar to equation (1), the techniques of thisdisclosure allow for any combination of parameters to be selected if theremaining parameters are known or are defined as constraints. Forexample, if the feedthrough capacitance 122, the resonant frequency, andthe inductance 124 are specified, the channel capacitance 128 may bedetermined based on equation (2).

In additional examples, a lumped or discrete inductor may be placed inelectrical network 120 in series with parasitic inductor 124 to producea combined inductance, which may be substituted into the parasiticinductance (L) of equations (1) and (2). By adding a lumped inductor toelectrical network 120, the range of capacitance values may be broughtinto a range more suitable for implementation.

In some examples, the resistance values for resistance 126 may beselected to control a magnitude of the resonance. In additionalexamples, an inductance 124 may be selected in conjunction withresistance 126 to control a Q-factor of the resonance. The Q-factor of aresonance may refer to how steeply or quickly the resonance rises and/orfalls across a range of frequencies (e.g., the slope of the resonance).For example, the Q-factor of the resonance may refer to center frequencyof the resonance divided by the bandwidth of the resonance. Thebandwidth of the resonance may be determined by finding the cutofffrequency on each side of the resonance. In some examples, the cutofffrequencies may be the frequencies at which the energy or power is 3 dBabove and/or below the resonance peak.

In further examples, one or more components within electrical network120 may be configured such that one or more of the following constraintsare satisfied: (1) an amount of energy reflected by electrical network120 along leads 18, 20, 22 is below a first threshold; (2) an amount ofenergy transferred by electrical network 120 to active circuitry 86 isbelow a second threshold; and/or (3) an amount of energy dissipated byelectrical network 120 is below a third threshold. The first thresholdmay be used to control lead heating effects produced when IMD 16 is inthe presence of or subject to energy emitted by an electromagneticenergy source. The second threshold may be used to control RFrectification that may occur when IMD 16 is in the presence of orsubject to energy emitted by an electromagnetic energy source. The thirdthreshold may be used to control device heating that may occur when IMD16 is in the presence of or subject to energy emitted by anelectromagnetic energy source.

In some examples, if any two constraints are selected, parameter valuesmay be configured to satisfy the remaining constraint. For example, ifthresholds are selected to control lead heating effects and RFrectification, then an amount of energy dissipated by electrical network120 may be configured such that the two thresholds are satisfied. Forexample, parasitic and/or actual resistances within electrical network120 may be adjusted to increase the amount of energy dissipated byelectrical network 120. In additional examples, if all three constraintsare specified, the techniques of this disclosure may be used to findcomponent parameters that satisfy all three constraints.

FIGS. 6A & 6B are charts 140, 160 illustrating an example reflectioncoefficient plot for electrical network model 120 according to thisdisclosure. Both of the charts 140, 160 include a frequency axis 142that increases from left to right, a reflection coefficient axis 144that increases from bottom to top, and a reflection coefficient plot 146for a range of frequencies. The reflection coefficient axis 146 isplotted in units of decibels (dB).

In general, the reflection coefficient for electrical network 120 may becalculated according to the following equation:

$\begin{matrix}{S = \frac{Z_{inp} - Z_{0}}{Z_{inp} + Z_{0}}} & (3)\end{matrix}$

where Z_(inp) represents the effective input impedance of electricalnetwork 120 (i.e., the effective impedance of electrical network 120when looking “into” input nodes 132A, 132B), Z₀ represents thecharacteristic impedance of the electrical lead, and S represents thereflection coefficient. In some examples, the reflection coefficient maycorrespond to the S₁₁ two-port network parameter.

The reflection coefficient plot shown in FIG. 6A has a resonance atfrequency f_(R). At the resonant frequency, the difference betweenZ_(inp) and Z₀ may be substantially reduced. In some examples, Z_(inp)and Z₀ may be substantially equal. The reduction in the differencebetween Z_(inp) and Z₀ may cause the amount of energy reflected alongthe lead by electrical network 120 to rapidly approach zero. In otherwords, at the resonant frequency, a substantial amount of the energy iseither transferred to active circuitry 86 or dissipated by electricalnetwork 120. Distances 148 and 150 illustrate two distances that may bedefined as magnitudes of the resonance. Distance 148 is defined as thedifference between the peak resonance reflection coefficient (S₁) andthe steady-state reflection coefficient for frequencies higher than theresonance (S₂). Distance 150 is defined as the difference between thepeak resonance reflection coefficient (S₁) and the steady-statereflection coefficient for frequencies lower than the resonance (S₃). Insome examples, the magnitude of the resonance may be defined as theaverage of distances 148, 150. In other examples, the magnitude of theresonance may be based on the value of the peak resonance reflectioncoefficient and some other reflection coefficient at a non-resonantfrequency (e.g., a cutoff frequency). According to the techniques inthis disclosure, resistance 126 may be configured and/or adjusted toachieve a desired magnitude for the resonance.

The reflection coefficient plot shown in FIG. 6B has two cutofffrequencies f₁ and f₂. The cutoff frequencies may be the frequencies atwhich the reflection coefficient is a certain level above or below thepeak resonance reflection coefficient (S₁). As shown in FIG. 6B, thecutoff frequencies are determined based on cutoff reflection coefficientvalue (S₄). In some examples, the cutoff reflection coefficient valuemay be 3 dB above and/or below the peak reflection coefficient at theresonant frequency. A bandwidth of the resonance may be determined byfinding the difference between the cutoff frequencies (f₂−f₁). Thequality factor (Q-factor) of a resonance may refer to center frequencyof the resonance (f_(R)) divided by the bandwidth of the resonance. TheQ-factor may refer to how steeply or quickly the resonance rises and/orfalls across a range of frequencies (e.g., a slope of the resonancecurve). According to the techniques in this disclosure, inductance 124may be configured and/or adjusted to achieve a desired Q-factor for theresonance.

FIG. 7 is a flow diagram illustrating an example technique for reducinglead heating effects according to this disclosure. According to FIG. 7,a frequency of electromagnetic wave is selected (180). The selectedfrequency may correspond to the frequency of an electromagnetic energysource that produces or emits EMI. The electromagnetic energy sourcemay, in some examples, be a medical imaging modality, such as, e.g., anMRI imaging modality. In some examples, the selected frequency may be arange of frequencies. In additional examples, rather than selecting afrequency of electromagnetic wave, a frequency of induced current may beselected.

A threshold that defines a maximum amount of reflected energy for theselected frequency may also be selected (182). The threshold maycorrespond to a level of reflected energy below which lead heatingeffects are acceptable when IMD 16 is placed in the presence of (e.g.,subject to) a disruptive energy field of a particular frequency. Inadditional examples, the threshold may define a maximum amount ofcurrent induced within a lead of IMD 16 when IMD 16 is placed in thepresence of a disruptive energy field.

A parasitic inductance value for a first component within IMD 16 isdetermined (184). In some examples, the first component may include oneor more ribbon bonds 80 within IMD 16. In some cases, the one or moreribbon bonds 80 may be laser ribbon bonds. In other examples, the firstcomponent may include one or more conductive traces 88, 90 within IMD16. In general, the first component may include any combination ofcomponents within IMD 16 that has a parasitic inductance.

A reactance for a second component within IMD 16 may be selected basedon the parasitic inductance value and the selected threshold (186). Forexample, the reactance of the second component may be selected based onthe parasitic inductance value such that an amount of energy reflectedalong the lead in response to an electromagnetic energy source is belowthe selected threshold. As another example, the reactance for the secondcomponent may be selected such that a resonance occurs at a frequencyproximate the selected frequency. By causing a resonance at a frequencyproximate to the selected frequency, the amount of energy reflectedalong the lead in response to an electromagnetic energy source may bereduced. In additional examples, the reactance of the second componentmay be selected based on the parasitic inductance value such that anamount of current induced within the lead is lead is below the selectedthreshold.

In some examples, the second component may be a capacitor, e.g. channelcapacitor 84, and the reactance may be a capacitance of the capacitor.In such examples, equation (1) may be used to select the appropriatevalue of channel capacitance based on the parasitic inductance and adesired resonant frequency. Equation (2) may be also used to select theappropriate value of channel capacitance based on the parasiticinductance, the capacitance of feedthrough capacitor 72, and a desiredresonant frequency.

FIG. 8 is a flow diagram illustrating another example technique forreducing lead heating effects according to this disclosure. Similar tothe technique shown in FIG. 7, a frequency of electromagnetic wave orinduced current is selected (188). A threshold that defines a maximumamount of reflected energy for the selected frequency may also beselected (190). Then, a parasitic inductance of a first component and areactance of a second component may be configured such that an amount ofreflected energy is less than the threshold (192). In some examples, thefirst component may include one or more ribbon bonds 80 and/or one ormore conductive traces 88, 90 within IMD 16. In such cases, theparasitic inductance of the first component may be configured byadjusting a length and/or width of the first component. For example, alength and/or width of ribbon bond 80 and/or conductive traces 88, 90may be configured to produce a desired parasitic inductance.

In some examples, an inductor may be placed in series with the firstcomponent. The combination of the actual inductance of the inductor andthe parasitic inductance of the first component may produce an effectiveinductance that can allow for a better selection of reactance values(e.g., capacitance values).

FIG. 9 is a flow diagram illustrating another example technique forreducing lead heating effects according to this disclosure. Similar tothe technique shown in FIG. 7, a frequency of electromagnetic wave orinduced current is selected (194). A first threshold that defines amaximum amount of reflected energy for the selected frequency may alsobe selected (196). A resonant frequency for an electrical network thatincludes a first component and a second component may be selected (198).In some examples, the resonant frequency may be selected such that theresonant frequency occurs at a frequency proximate to or substantiallyequal to the selected frequency of electromagnetic wave. Then, theparasitic inductance of a first component and the reactance of a secondcomponent may be configured such that the electrical network operates inresonance at the resonant frequency (200).

A second threshold may be determined based on the first threshold (202).The second threshold may define a maximum amount of reflected energy atthe resonant frequency. For example, the second threshold may define aresonance magnitude that is needed to achieve the reflected energy levelspecified by the first threshold. Then, a resistance within theelectrical network may be configured such that the amount of energyreflected along the lead at the resonant frequency is less than thesecond threshold. (204).

In some examples, the resistance configured in step 204 may be aparasitic resistance of the first component. In another example, theresistance configured in step 204 may be an effective input resistance(i.e., real input impedance) for electrical network 120.

In additional examples, the parasitic inductance of the first componentmay be adjusted in addition to or in lieu of the resistance to cause theamount of energy reflected along the lead at the resonant frequency tobe less than the second threshold. The parasitic inductance of the firstcomponent may be configured to cause the electrical network 120 to havea particular bandwidth or Q-factor, which will in turn allow the desiredamount of reflected energy to be achieved. For example, when the firstcomponent is a conductor, the length and/or width may be configured toproduce a particular parasitic inductance.

FIG. 10 is a flow diagram illustrating an example technique forcontrolling lead heating effects and power rectification according tothis disclosure. A frequency of electromagnetic wave is selected (206).A first threshold that defines a maximum amount of reflected energy forthe selected frequency may also be selected (208). A second thresholdthat defines a maximum amount of energy transferred into IMD 16 may beselected for the selected frequency (210). A third threshold thatdefines a minimum amount of energy dissipated by an electrical networkmay be determined based on the first and second thresholds (212). One ormore components within the electrical network may be configured suchthat an amount of energy dissipated by the electrical network is greaterthan the third threshold (214). The one or more components may includethe first and second components described above with respect to FIG. 7.

FIG. 11 is a flow diagram illustrating an example technique forcontrolling lead heating effects and device heating effects according tothis disclosure. A frequency of electromagnetic wave is selected (216).A first threshold that defines a maximum amount of reflected energy forthe selected frequency may also be selected (218). A second thresholdthat defines a maximum amount of energy dissipated by an electricalnetwork may be selected (220). A third threshold that defines a minimumamount of energy transferred into IMD 16 may be determined based on thefirst and second thresholds (222). One or more components within theelectrical network may be configured such that an amount of energytransferred into IMD 16 is greater than the third threshold (224). Theone or more components may include the first and second componentsdescribed above with respect to FIG. 7.

FIG. 12 is a flow diagram illustrating an example technique forcontrolling lead heating effects, power rectification, and deviceheating effects according to this disclosure. A frequency ofelectromagnetic wave is selected (226). A first threshold that defines amaximum amount of reflected energy for the selected frequency may alsobe selected (228). A second threshold that defines a maximum amount ofenergy dissipated by an electrical network may be selected (230). Athird threshold that defines a maximum amount of energy transferred intoIMD 16 may also be selected (232). One or more components within theelectrical network may be configured such that all three thresholds aresatisfied (234).

With regard to the flow diagrams in FIGS. 7-12, it should be noted thatthe ordering of steps in the flow diagram are merely exemplary, and thatother orderings are within the scope of this disclosure. In addition, itshould be recognized that steps may be removed and/or deleted from thediagrams without departing from this disclosure.

In some examples, the thresholds described with respect to FIGS. 7-12 ofthis disclosure may refer to a voltage or current amplitudes as opposedto an amount of energy, and the components may be configured such thatthe voltage or current amplitudes are below the selected threshold. Ingeneral, any metric associated with the reflected wave, lead heatingeffects, RF rectification, and device heating effects may be used as thedesign criteria for the parasitic inductance and reactance withoutdeparting from the scope of this disclosure.

The techniques of this disclosure may be implemented by an IMD that isconfigured to provide pacing therapy, and/or cardio-version shocks. Inaddition, the techniques in this disclosure may also be applied to othertypes of IMDs. For example, the techniques in this disclosure may beapplied to neurostimulators, including deep brain stimulators, spinalcord stimulators, peripheral nerve stimulators, pelvic floorstimulators, gastro-intestinal stimulators, or the like.

The techniques described in this disclosure, including those attributedto image IMD 16, programmer 24, or various constituent components, maybe implemented, at least in part, in hardware, software, firmware or anycombination thereof. For example, various aspects of the techniques maybe implemented within one or more processors, including one or moremicroprocessors, digital signal processors (DSPs), application specificintegrated circuits (ASICs), field programmable gate arrays (FPGAs), orany other equivalent integrated or discrete logic circuitry, as well asany combinations of such components, embodied in programmers, such asphysician or patient programmers, stimulators, image processing devicesor other devices. The term “processor” or “processing circuitry” maygenerally refer to any of the foregoing logic circuitry, alone or incombination with other logic circuitry, or any other equivalentcircuitry.

Such hardware, software, firmware may be implemented within the samedevice or within separate devices to support the various operations andfunctions described in this disclosure. In addition, any of thedescribed units, modules or components may be implemented together orseparately as discrete but interoperable logic devices. Depiction ofdifferent features as modules or units is intended to highlightdifferent functional aspects and does not necessarily imply that suchmodules or units must be realized by separate hardware or softwarecomponents. Rather, functionality associated with one or more modules orunits may be performed by separate hardware or software components, orintegrated within common or separate hardware or software components.

When implemented in software, the functionality ascribed to the systems,devices and techniques described in this disclosure may be embodied asinstructions on a computer-readable medium such as random access memory(RAM), read-only memory (ROM), non-volatile random access memory(NVRAM), static RAM (SRAM), electrically erasable programmable read-onlymemory (EEPROM), FLASH memory, magnetic data storage media, optical datastorage media, or the like. The instructions may be executed to supportone or more aspects of the functionality described in this disclosure.

Various examples have been described. These and other examples arewithin the scope of the following claims.

The invention claimed is:
 1. An implantable medical system comprising:an implantable lead that includes one or more electrodes; an implantablemedical device (IMD) coupled to the implantable lead, the implantablemedical device comprising: a first component electrically coupled to thelead, the first component having a parasitic inductance; and a secondcomponent electrically coupled to the lead, the second component havinga reactance, wherein the parasitic inductance of the first component andthe reactance of the second component are selected based on a frequencyof energy emitted by an electromagnetic energy source and a selectedthreshold such that an amount of energy reflected along the lead inresponse to the frequency of the energy emitted by the electromagneticenergy source is below the selected threshold.
 2. The system of claim 1,wherein the first component comprises at least one of a ribbon bond anda conductive trace.
 3. The system of claim 1, wherein the IMD furthercomprises: a feedthrough assembly; and a circuit board coupled to thefeedthrough assembly via the first component.
 4. The system of claim 1,wherein the IMD further comprises: a circuit board having a pad and achannel capacitor disposed thereon, wherein the channel capacitor iscoupled to the pad via the first component.
 5. The system of claim 1,wherein the IMD further comprises: an inductor coupled in series withthe first component, wherein the parasitic inductance for the firstcomponent and the reactance of the second component are selected basedon the inductance of the inductor.
 6. The system of claim 1, wherein theIMD further comprises: a circuit board comprising: a pad disposed on thecircuit board; and a diode disposed on the circuit board and coupled tothe pad via a conductive trace, wherein the second component comprises acapacitor coupled between the conductive trace and a ground voltage forthe circuit board.
 7. The system of claim 6, wherein the first componentcomprises the conductive trace.
 8. The system of claim 6, wherein theIMD further comprises: a feedthrough assembly; and a ribbon bond coupledbetween the feedthrough assembly and the pad, wherein the firstcomponent comprises the ribbon bond.
 9. The system of claim 6, whereinthe IMD further comprises: a housing; a conductive feedthrough pin; anda feedthrough capacitor coupled between the conductive feedthrough pinand the housing of the IMD.
 10. The system of claim 9, wherein theparasitic inductance of the first component and the capacitance of thefeedthrough capacitor are selected based on the reactance of the secondcomponent.
 11. The system of claim 1, wherein the selected thresholddefines a maximum amount of energy reflected by the IMD when the IMD issubject to the energy produced by the electromagnetic energy source. 12.The system of claim 11, wherein the selected threshold is a firstthreshold, wherein the parasitic inductance of the first component andthe reactance of the second component are selected such that an amountof energy reflected along the lead in response to the energy produced bythe electromagnetic energy source is below the first threshold and anamount of energy transferred to a diode is below a second threshold, andwherein the second threshold is based on a maximum amount of energytransferred to the diode.
 13. The system of claim 11, wherein theselected threshold is a first threshold, wherein the parasiticinductance of the first component and the reactance of the secondcomponent are selected such that an amount of energy reflected along thelead in response to the energy produced by the electromagnetic energysource is below the first threshold and an amount of energy dissipatedby an electrical network that includes the first component and thesecond component is below a second threshold.
 14. The system of claim 1,wherein the parasitic inductance of the first component and thereactance of the second component are selected such that the firstcomponent and the second component produce a resonance at a resonantfrequency proximate to a frequency of the energy produced by theelectromagnetic energy source.
 15. The system of claim 14, wherein theselected threshold is a first threshold, wherein the first thresholddefines a maximum amount of energy reflected by the IMD at the frequencyof the energy produced by the electromagnetic energy source, wherein aparasitic resistance of the first component is selected such that anamount of energy reflected along the lead at the resonant frequency isless than a second threshold, and wherein the second threshold defines amaximum amount of energy reflected along the lead at the resonantfrequency based on the first threshold.
 16. The system of claim 15,wherein the parasitic resistance and parasitic inductance of the firstcomponent are selected such that the amount of energy reflected alongthe lead at the resonant frequency is less than the second threshold.17. The system of claim 1, wherein the electromagnetic energy sourcecomprises an imaging modality.
 18. The system of claim 17, wherein theimaging modality comprises a magnetic resonance imaging (MRI) modality.19. The system of claim 1, wherein a length of the first component isselected such that the amount of energy reflected along the lead inresponse to the frequency of energy emitted by the electromagneticenergy source is below the selected threshold.
 20. The system of claim1, wherein the IMD comprises an implantable cardiac device.
 21. A methodcomprising: selecting a parasitic inductance of a first component and areactance of a second component within an implantable medical device(IMD) based on a frequency of energy emitted by an electromagneticenergy source and a selected threshold such that an amount of energyreflected along a lead of the IMD in response to the frequency of theenergy being emitted by the electromagnetic source is below the selectedthreshold.
 22. The method of claim 21, wherein the first componentcomprises at least one of a ribbon bond and a conductive trace.
 23. Themethod of claim 21, wherein the first component is coupled between afeedthrough assembly of the IMD and a circuit board contained within theIMD.
 24. The method of claim 21, wherein the first component is coupledbetween a pad disposed on a circuit board contained within the IMD and achannel capacitor disposed on the circuit board.
 25. The method of claim21, wherein an inductor is coupled in series with the first component,and wherein selecting the parasitic inductance of the first componentcomprises selecting the parasitic inductance of the first componentbased on the reactance of the second component and an inductance valueof the inductor.
 26. The method of claim 21, wherein the IMD comprises acircuit board having a pad, a diode, and a conductive trace disposedthereon, wherein the conductive trace is coupled between the pad and thediode, and wherein the second component is a capacitor coupled betweenthe conductive trace and a ground voltage for the circuit board.
 27. Themethod of claim 26, wherein the first component comprises the conductivetrace.
 28. The method of claim 26, wherein the IMD further comprises afeedthrough assembly and a ribbon bond coupled between the feedthroughassembly and the pad, and wherein the first component comprises theribbon bond.
 29. The method of claim 26, wherein the IMD furthercomprises a housing and a conductive feedthrough pin, wherein afeedthrough capacitor is coupled between the conductive feedthrough pinand the housing of the IMD.
 30. The method of claim 29, whereinselecting the parasitic inductance of the first component comprisesselecting the parasitic inductance of the first component and selectingthe capacitance of the feedthrough capacitor based on the reactance ofthe second component.
 31. The method of claim 21, wherein the selectedthreshold defines a maximum amount of energy reflected by the IMD whenthe IMD is subject to the energy produced by the electromagnetic energysource.
 32. The method of claim 31, wherein the threshold is a firstthreshold, wherein the method further comprises selecting a secondthreshold that defines a maximum amount of energy transferred to a diodewithin the IMD, and wherein selecting the parasitic inductance of thefirst component and the reactance of the second component comprisesselecting the parasitic inductance of the first component and thereactance of the second component such that an amount of energyreflected along the lead in response to the energy produced by theelectromagnetic energy source is below the first threshold and an amountof energy transferred to the diode is below the second threshold. 33.The method of claim 31, wherein the threshold is a first threshold,wherein the method further comprises selecting a second threshold thatdefines a maximum amount of energy dissipated by an electrical networkthat includes the first component and the second component, and whereinselecting the parasitic inductance of the first component and thereactance of the second component comprises selecting the parasiticinductance of the first component and the reactance of the secondcomponent such that an amount of energy reflected along the lead inresponse to the energy produced by the electromagnetic energy source isbelow the first threshold and an amount of energy dissipated by anelectrical network that includes the first component and the secondcomponent is below the second threshold.
 34. The method of claim 21,wherein selecting the parasitic inductance of the first componentcomprises selecting the parasitic inductance such that the firstcomponent and the second component produce a resonance at a resonantfrequency proximate to a frequency of the energy produced by theelectromagnetic energy source.
 35. The method of claim 34, wherein theselected threshold is a first threshold, and wherein the method furthercomprises: selecting the first threshold to define a maximum amount ofenergy reflected by the IMD at the frequency of the energy produced bythe electromagnetic energy source; determining a second threshold thatdefines a maximum amount of energy reflected along the lead at theresonant frequency based on the first threshold; and selecting aparasitic resistance of the first component such that the amount ofenergy reflected along the lead at the resonant frequency is less thanthe second threshold.
 36. The method of claim 35, wherein selecting theparasitic resistance of the first component comprises selecting theparasitic resistance and the parasitic inductance of the first componentsuch that the amount of energy reflected along the lead at the resonantfrequency is less than the second threshold.
 37. The method of claim 21,wherein the electromagnetic energy source comprises an imaging modality.38. The method of claim 37, wherein the imaging modality comprises amagnetic resonance imaging (MRI) modality.
 39. The method of claim 21,wherein selecting the parasitic inductance comprises selecting a lengthof the first component.
 40. The method of claim 21, wherein the IMDcomprises an implantable cardiac device.